As widely known, piezoelectric materials have the property of expanding and contracting upon voltage application. To apply the property to industrial use, there have been developed piezoelectric elements including a first electrode, a second electrode and a piezoelectric body sandwiched between the first and second electrodes. The piezoelectric elements have been used for ink discharge actuators in inkjet printers, magnetic head drive actuators in hard disk drives, as well as micropump drive actuators.
FIG. 9 is a schematic sectional view illustrating a basic piezoelectric element 101. The piezoelectric element 101 is prepared by forming a piezoelectric body 102 into a film and arranging electrodes 103 and 104 on the upper and lower surfaces of the piezoelectric body 102, respectively. In general, the piezoelectric body 102 is about 1 μm to 100 μm in thickness and the lower electrode 104 is larger in thickness than the upper electrode 103 to have greater rigidity than the upper electrode 103. The piezoelectric body 102 shows spontaneous polarization oriented toward the upper electrode 103.
As shown in FIG. 10 the piezoelectric body 102 expands and contracts in the horizontal direction when a voltage is applied to both of the electrodes 103 and 104 such that the upper electrode 103 functions as a positive electrode. Since the lower electrode 104 is greater in rigidity than the upper electrode 103 as described above, part of the piezoelectric element 101 near the lower electrode 104 bulges, while part of the piezoelectric element 101 near the upper electrode 103 is dented.
To deform the piezoelectric element 101 as described above, the two electrodes 103 and 104 need to have different values of rigidity. The electrodes 103 and 104 are also required to be thinned down to such a degree that the piezoelectric body 102 is warped. Therefore, in general, the electrodes 103 and 104 are several nm to several μm in thickness.
There is a wide range of variations of materials for the piezoelectric body. Among them, a piezoelectric material containing a lead compound is industrially useful for its large piezoelectric constant. Examples of the lead compound-containing piezoelectric material include lead titanate (PT), lead zirconium titanate (PZT), as well as PZT added with magnesium, manganese, cobalt, iron, nickel, niobium, scandium, tantalum, bismuth or tantalum.
In general, the piezoelectric body has a polycrystalline structure. The higher crystallinity the piezoelectric body has, the more useful it is in industrial application because the directions of spontaneous polarization are more likely to be aligned and the piezoelectric constant becomes larger. Therefore, aiming at an improvement in crystallinity of the piezoelectric body, various methods for manufacturing the piezoelectric element have been studied. The piezoelectric elements formed by these manufacturing methods have improved in crystallinity. However, there has not been realized a perfect monocrystalline piezoelectric body in a large-area piezoelectric element. In inkjet printers and hard disk drives, piezoelectric elements have as large area as several hundred μm2, and therefore they contain a number of grain boundaries therein.
The higher the field intensity applied to the piezoelectric body is, the higher the degree of deformation of the piezoelectric element becomes. Therefore, in some cases, field intensity as high as 104 V/cm or more is applied. However, when a high voltage is applied to the lead-containing piezoelectric body in a high humidity environment, leakage current increases to cause dielectric breakdown.
It has not been clear on which principle dielectric breakdown occurs upon high voltage application in a high humidity environment. However, it is presumed that a cause of the dielectric breakdown is leakage current caused by moisture which seeps into grain boundaries and miniscule pinholes in the piezoelectric body. If the electrodes sandwiching the piezoelectric body are thick enough, the electrodes function as barriers to prevent moisture from seeping into the piezoelectric body. However, to make the piezoelectric element work, the thickness of the electrodes cannot be increased as described above. As a result, moisture passes through the pinholes in the electrodes to seep into the grain boundaries in the piezoelectric body, thereby causing dielectric breakdown.
Thus, it is presumed that modification to the grain boundaries and pinholes in the piezoelectric body allows an improvement in insulation reliability. Hereinafter, reference is made to examples of a conventional method for making modification to the grain boundaries and pinholes in the piezoelectric body. In this specification, provided that the piezoelectric body is an aggregate of a plurality of crystals (may be referred to as crystal grains), the grain boundary mentioned herein is a boundary between adjacent crystals. Although mathematically defined boundaries are faces having no line and thickness, the boundary mentioned herein includes faces having thicknesses. More specifically, a gap between adjacent crystals is referred to as the grain boundary. In this gap, other substances than the crystals exist. “Other substances than the crystals” are substances having structures and elemental composition different from those of the crystals. In some cases, the gap is completely filled with the other substances than the crystals, or in other cases it is partially filled with the other substances to leave space therein. When the space is left therein, in general, part or all of the surfaces of the crystals is covered with the other substances than the crystals. Further, the pinholes are holes which penetrate the piezoelectric body. If the gap between adjacent crystals penetrates the piezoelectric body, the grain boundary is a pinhole. Therefore, in a broad sense, the pinholes are included in the grain boundaries.
Japanese Patent No. 3206454 (hereinafter referred to as Patent Literature 1) discloses a method of sealing pinholes in composite oxide, such as PZT prepared by hydrosynthesis, with a resin or ceramic having a high dielectric constant. More specifically, the surface of the composite oxide is coated, sprayed or impregnated with a liquid prepared by dissolving a precursor of a resin material or ceramic in a solvent to fill the pinholes in the composite oxide with the liquid. Then, the liquid is solidified by drying or sintering.
Patent Literature 1 further discloses a method of sealing pinholes in composite oxide formed by hydrosynthesis on a metal substrate by immersing the metal substrate in an aqueous oxidizing solution and applying electric current thereto. In this method, the aqueous oxidizing solution passes through the pinholes in the composite oxide to contact the metal substrate, thereby causing an electrochemical reaction. As a result, the metal surface in the pinholes is turned to be insulating oxide, thereby sealing the pinholes.
Japanese Unexamined Patent Publication No. 10-217458 (hereinafter referred to as Patent Literature 2) discloses a piezoelectric element including a piezoelectric body sandwiched between two electrodes, wherein a dielectric material having a lower dielectric constant than that of the piezoelectric body exists in a grain boundary exposing region. Where the piezoelectric element is configured as described above, an electric field applied to the grain boundary upon voltage application between the electrodes is reduced to a greater extent than where nothing exists in the grain boundary exposing region. As a result, dielectric breakdown derived from the grain boundary is prevented.
However, the method of filling the pinholes in the composite oxide with an insulating material disclosed by Patent Literature 1 is defective as described below. More specifically, the density of the solid obtained after drying and sintering increases with an increase in concentration of the solid substance in the liquid used. This enhances the effect of sealing the pinholes. On the other hand, the increase in concentration of the solid substance also brings about an increase in viscosity of the liquid. Therefore, the liquid becomes less prone to go into the pinholes, whereby the sealing effect is reduced. Thus, this method has difficulty in completely sealing the pinholes.
Further, according to the method of filling the pinholes in the composite oxide formed on the metal substrate disclosed by Patent Literature 1, it is hard to produce the insulating oxide if the metal substrate is made of noble metal such as gold or platinum. Therefore, there is only a limited choice of usable materials for the metal substrate, such as titanium and aluminum.
In the method disclosed by Patent Literature 2, the dielectric material lies on the surface where the grain boundary is exposed. This reduces the voltage applied to the grain boundary, but a certain voltage is still applied to the grain boundary. Therefore, leakage current cannot be fully prevented.
Under these circumstances, the inventors of the present invention have conducted additional studies on the principle of how dielectric breakdown occurs upon high voltage application in a high humidity environment. As a result, they have found that a main cause of the dielectric breakdown in a high humidity environment is the deterioration of lead oxide present at the grain boundaries in the piezoelectric body caused by an electrochemical reaction between lead oxide and moisture.
Hereinafter, the details of the principle are described. FIG. 11 shows a sectional view of a piezoelectric element 101 including a lead compound-containing piezoelectric body 102 sandwiched between two electrodes 103 and 104. In the piezoelectric element 101, the upper electrode 103 is formed thinner than the lower electrode 104 such that part of the piezoelectric element 101 near the lower electrode 104 bulges when a voltage is applied across the electrodes 103 and 104. Accordingly, a number of pinholes 103a are formed in the upper electrode 103. In a high humidity environment, moisture passes through the pinholes 103a in the upper electrode 103 to seep into grain boundaries 102b between columnar crystals 102a. Insulating PbO (lead oxide) present at the grain boundaries 102b reacts with seeping moisture to become Pb(OH)2 (lead hydroxide) (Pb(OH)2 is indicated as PbOH in FIG. 12; the same is also applied to FIG. 13). As shown in FIGS. 13(a) and 13(b), part of Pb(OH)2 near the positive electrode is oxidized to PbO2 (lead dioxide) having as high conductivity as that of metal. Further, as shown in FIGS. 13(b) and 13(c), the resulting PbO2 functions as the positive electrode to oxidize adjacent part of Pb(OH)2 to PbO2. Then, eventually, the positive and negative electrodes are electrically connected via PbO2 as shown in FIG. 13(d) to cause dielectric breakdown.
Thus, the inventors of the present invention have found that a piezoelectric element having high insulation reliability is realized by eliminating the above-described cause of dielectric breakdown.
In view of the above, the present invention has been achieved. An object of the present invention is to provide a technique of improving the insulation reliability of a piezoelectric element including a first electrode, a second electrode and a lead compound-containing containing piezoelectric body sandwiched between the first and second electrodes.